The present invention relates to an improved process for preparing a tubular film of hydrolyzed ethylene-vinyl acetate copolymer.
The hydrolyzed ethylene-vinyl acetate copolymer has a higher impermeability for oxygen than other thermoplastic resins and also has the excellent antistatic property and oil resistance. Therefore, the film of said copolymer has been expected as wrapping or packaging materials for foods and drugs.
In molding the film of the hydrolyzed ethylene-vinyl acetate copolymer being a kind of thermoplastic resin, both of tubular extrusion and flat film extrusion have been proposed like other known thermoplastic resins, but the tubular extrusion has not commercially employed yet.
The present invention relates to a tubular extrusion.
The term "tubular extrusion" as used herein means a typical process which comprises extruding the copolymer through a circular die, followed by expansion by the pressure of internal air admitted through the center of a mandrel, collapsing of the bubble and winding up through the nip rolls, and is called "blow-extrusion process" or "inflation process", too.
This process has been commercially employed for polyethylene, polypropylene, and so on.
Many attempts have been made by the present inventors to prepare a tubular film of the hydrolyzed ethylene-vinyl acetate copolymer on the basis of known tubular extrusion for polyethylene, but optimum conditions for the resin are not suitable for the copolymer. For instance, when the tubular film is extruded at the same condition to polyethylene, a large number of wrinkles occurs on passing the nip rolls. Also when the tubular film is drawn by nip rolls after biaxially stretching it, the film is often teared off. Such troubles upon passing through nip rolls are not seen in the preparation of a tubular film of other thermoplastic resins such as polyethylene and polypropylene. Continual attempts have therefore been made to eliminate such troubles under the notion that they are caused by the physical properties peculiar to the hydrolyzed ethylene-vinyl acetate copolymer.